Electrically conductive organic polymers emerged in the 1970's as a new class of electronic materials. These materials have the potential of combining the electronic and magnetic properties of metals with the light weight, processing advantages, and physical and mechanical properties characteristic of conventional organic polymers. Examples of electrically conducting polymers are polyparaphenylene vinylenes, polyparaphenylenes, polyanilines, polytbiophenes, polyazines, polyfuranes, polythianaphthenes polypyrroles, polyselenophenes, poly-p-phenylene sulfides, polyacetylenes formed from soluble precursors, combinations thereof and blends thereof with other polymers and copolymers of the monomers thereof.
These polymers are conjugated systems which are made electrically conducting by doping. The doping reaction can involve an oxidation, a reduction, a protonation, an alkylation, etc. The non-doped or non-conducting form of the polymer is referred to herein as the precursor to the electrically conducting polymer. The doped or conducting form of the polymer is referred to herein as the “conducting polymer.”
Conducting polymers have potential for a large number of applications in such areas such as electrostatic charge/discharge (ESC/ESD) protection, electromagnetic interference (EMI) shielding, resists, electroplating, corrosion protection of metals, and ultimately metal replacements, i.e. wiring, plastic microcircuits, conducting pastes for various interconnection technologies (solder alternative), etc. Many of the above applications especially those requiring high current capacity have not yet been realized because the conductivity of the processible conducting polymers is not yet adequate for such applications.
To date, polyacetylene exhibits the highest conductivity of all the conducting polymers. The reason for this is that polyacetylene can be synthesized in a highly crystalline form (crystallinity as high as 90% has been achieved) (as reported in Macromolecules, 25, 4106, 1992). This highly crystalline polyacetylene has a conductivity on the order of 105 S/cm. Although this conductivity is comparable to that of copper, polyacetylene is not technologically applicable because it is a non-soluble, non-processible, and environmentally unstable polymer.
The polyaniline class of conducting polymers has been shown to be probably the most suited of such materials for commercial applications. Great strides have been made in making the material quite processable. It is environmentally stable and allows chemical flexibility which in turn allows tailoring of its properties. Polyaniline coatings have been developed and commercialized for numerous applications. Devices and batteries have also been constructed with this material. However, the conductivity of this class of polymers is generally on the low end of the metallic regime. The conductivity is on the order of 100 S/cm. Some of the other soluble conducting polymers such as the polythiophenes, poly-para-phenylenevinylenes exhibit conductivity on the order of 102 S/cm. It is therefore desirable to increase the conductivity of the soluble/processible conducting polymers, in particular the polyaniline materials.
The conductivity (σ) is dependent on the number of carriers (n) set by the doping level, the charge on the carriers (q) and on the interchain and intrachain mobility (μ) of the carriers.σ=nqμ
Generally, n (the number of carriers) in these systems is maximized and thus, the conductivity is dependent on the mobility of the carriers. To achieve higher conductivity, the mobility in these systems needs to be increased. The mobility, in turn, depends on the morphology of the polymer. The intrachain mobility depends on the degree of conjugation along the chain, presence of defects, and on the chain conformation. The interchain mobility depends on the interchain interactions, the interchain distance, the degree of crystallinity, etc. Increasing the crystallinity results in increased conductivity as exemplified by polyacetylene. To date, it has proven quite difficult to attain polyaniline in a highly crystalline state. Some crystallinity has been achieved by stretch orientation or mechanical deformation (A. G. MacDiamid et al in Synth. Met. 55-57, 753). In these stretch-oriented systems, conductivity enhancements have been observed. The conductivity enhancement was generally that measured parallel to the stretch direction. Therefore, the conductivity in these systems is anisotropic. It is desirable to achieve a method of controlling and tuning the morphology of polyaniline. It is desirable to achieve a method of controlling and tuning the degree of crystallinity and the degree of amorphous regions in polyaniline, which in turn provides a method of tuning the physical, mechanical, and electrical properties of polyaniline. It is further desirable to achieve highly crystalline and crystalline polyaniline and to achieve this in a simple and useful manner in order to increase the mobility of the carriers and, therefore, the conductivity of the polymer. It is also further desirable to achieve isotropic conductivity, that is conductivity not dependent on direction as with stretch-oriented polyanilines.